Effects of influent C/N ratios and treatment technologies on integral biogas upgrading and pollutants removal from synthetic domestic sewage

Three different treatment technologies, namely mono-algae culture, algal-bacterial culture, and algal-fungal culture, were applied to remove pollutants form synthetic domestic sewage and to remove CO2 from biogas in a photobioreactor. The effects of different initial influent C/N ratios on microalgal growth rates and pollutants removal efficiencies by the three microalgal cultures were investigated. The best biogas upgrading and synthetic domestic sewage pollutants removal effect was achieved in the algal-fungal system at the influent C/N ratio of 5:1. At the influent C/N ratio of 5:1, the algal-fungal system achieved the highest mean chemical oxygen demand (COD) removal efficiency of 81.92% and total phosphorus (TP) removal efficiency of 81.52%, respectively, while the algal-bacterial system demonstrated the highest mean total nitrogen (TN) removal efficiency of 82.28%. The average CH4 concentration in upgraded biogas and the removal efficiencies of COD, TN, and TP were 93.25 ± 3.84% (v/v), 80.23 ± 3.92%, 75.85 ± 6.61%, and 78.41 ± 3.98%, respectively. These results will provide a reference for wastewater purification ad biogas upgrading with microalgae based technology.

In previous studies, significant improvement in the removal of xenobiotics from wastewaters has been observed with algae-bacteria/fungus-algae co-cultivation 14,15 . The success of this technology is dependent on a mutually beneficial relationship, that is, by fixing CO 2 which is produced by fungi/bacteria, algae can synthesize carbon source in the form of sugars and nutrients for fungi/bacteria through photosynthesis 16 . Therefore, pretreating wastewater by algal-fungal culture or algal-bacterial culture has an advantage on higher pollutants removal efficiencies comparing with mono-cultivation of algae cells, thereby offering a promising and efficient way to treat wastewater.
Biogas represents a renewable energy source based on its high CH 4 content. Raw biogas usually consists of methane (47-65%), carbon dioxide (34-41%), and other trace compositions including hydrogen sulfide, water vapor, etc. 17 . However, the CH 4 content of more than 90% (v/v) in biogas is required for vehicle fuel according to the natural gas standard 7 . The relatively high content of CO 2 in raw biogas will lower its heat content as well as increase its energy demand for compression and transportation. Therefore, CO 2 removal is essential for biogas upgrading. Numerous methods, such as physical absorption, chemical conversion, membrane separation, pressure swing adsorption, and cryogenic separation, have been used to upgrade biogas 17 . However, these methods require large amounts of energy, auxiliary materials, and chemicals, as well as generate wastes that require further treatment 18 . Moreover, the CO 2 separated from the raw biogas using the above-mentioned methods is usually released directly into the atmosphere and result in greenhouse gas emission. Among the various strategies for biogas upgrading by CO 2 removal, the biological sequestration of CO 2 using photosynthetic microalgae has been receiving considerable attention because of the relatively high CO 2 fixation capability of microalgae 11 . There are a limited number of studies on the integration of digestate treatment procedures and biogas upgrading via microalgae culture systems 8 . Accordingly, using microalgal growth to remove CO 2 from crude biogas is an economic and efficient method for biogas upgrading, which makes biogas upgrading by microalgal culturing an economically convenient technique when used in conjunction with wastewater treatment 8,19 .
Microalgal biomass production exhibits potentials for both pollutants reduction and CO 2 removal. However, microalgal growth is also affected by organic matter and nutrient concentrations in wastewater as well as the presence of other heterotrophic microorganisms 20 . Fungi and bacteria can also have strong degradation abilities for certain wastewater contaminants and can be associated with microalgae to form immobilization systems of algal-fungal culture or algal-bacterial culture with multiple functions. The advantages of these associations in wastewater treatment are: (i) improved collection of biomass from wastewaters, (ii) easily recycled and manipulated consortia, (iii) improved features of microalgae such as thermal stability and productivity, and (iv) harvestable bioresource from proliferated microalgae biomass 19,21,22 . The use of biogas and sewage as raw materials can not only output high-grade biogas through the microalgae photosynthesis to assimilate carbon dioxide, but also can purify sewage by accumulating C, N, and P in sewage with microalgae. When compared with the traditional biogas upgrading and sewage purification technology, environmental friendly and cost-effective are the biggest advantages. Some researches on biogas upgrading or sewage purification resulted in methane loss and energy consumption, or lead to the increase of effluent 18 . Microalgae-based technology was therefore developed for simultaneously treating wastewater and upgrading biogas in recently years. Combination of wastewater treatment and biogas upgrading is indispensable because of its highly economically convenient comparing with wastewater treatment by using microalgae only 23 . Besides, the high cost generated from microalgae biomass harvesting is one of the major bottlenecks for the industrialization of algae-based technology because of the small microalgal size, negative surface charge, and low biomass concentration of microalgae 24 . Co-culture of microalgae with fungi/bacteria can well solve this problem. The bio-flocculation based microalgae assisted with fungi/bacteria is regarded as one of the best ways to realize the large-scale separation and microalgae recovery 25 . The energy consumption and the cost of the subsequent enrichment process have been significantly reduced 14 . At present, the few studies on couple biogas upgrading with wastewater treatment in photobioreactors provide little information on the influence of operational conditions on strain composition. Influent C/N ratio is the main factor affecting biological wastewater treatment processes 26 . However, thus far, only a few studies have evaluated the effects of different influent C/N ratios on the efficiency of domestic sewage treatment by microalgae [27][28][29] . Overlow or overhigh C/N ratios in the influent may result in low growth rate of microalgae and low removal efficiencies of nitrogen and phosphate in wastewater. In addition, the C/N ratio is also vital to the growth of microalgae when cocultivation with fungi or bacteria.
This study proposes an integrated approach for synthetic domestic sewage treatment and biogas upgrading through three algal culture methods (mono-algal culture, algal-fungal culture, and algal-bacterial culture). The purposes of this study were to (1) evaluate the effects of three different microalgae culture methods on sewage purification and biogas upgrading; and (2) identify the optimal influent C/N ratio to achieve the highest efficiencies of pollutants removal combined with biogas upgrading in synthetic domestic sewage.

Results and Discussion
Physicochemical variations. Changes in values of the sewage pH and DO are shown in Table 1. These changes were similar under all tested influent C/N ratios. The pH ranged from 7.11 to 7.54 in the influent, which were optimum values for the microalgae cultivation and the prevention of ammonia toxicity and phosphate precipitation 30 . No significant difference (p > 0.05) was observed among the three algae source cultures for pH values in the influent or effluent. However, for all treatments, pH of the influent was significantly higher (p < 0.05) than that in the effluent, which was typically below pH 8.0 (Table 1). The pH values observed in this study were consistent with those found by Papazi et al. (2008), who reported that pH values slightly varied between 6.0 and 7.5 under CO 2 30% (v/v) with Chlorella minutissima 31 . This phenomenon meant that the biogas CO 2 content in this research met the requirement for microalgae growth. Therefore, the CO 2 consumption of the suspension culture only slightly affected the pH, which was rather related to the removal of organic pollutants. For instance, the reduced pH partly depends on the extent of nitrification in the photobioreactor, since ammonium volatilization processes play a negligible role in TN removal 13 .
The DO of the wastewater slightly decreased during the experimental period. The biggest decrease varied from 8.12 ± 2.11 mg L −1 to 6.09 ± 1.17 mg L −1 in the C2.5N1-COD100 treatment by algal-fungal culture. However, this variation ranged below the excessive DO level (35 mg L −1 , thereby would not inhibit microbial growth 32 . Carvalho et al. (2006) have reported that in a closed photobioreactor (similar to the photobioreactor used in this study), O 2 in the influent might inhibit microalgal growth because of photorespiration 33 . This phenomenon did not occur in this research because of relatively high initial CO 2 concentrations (28.43 ± 3.04%, v/v) in the photobioreactor and high organic carbon values in the influent 34 . Growth of the three selected cultures at various influent C/N ratios. Biomass productivity is a key parameter to analyze the potential of the three selected cultures to remove CO 2 . It generally varies with operational factors, such as light intensity, pH, working volume of the photobioreactor, and initial CO 2 concentration in the simulated biogas 20 . Studies have found that biomass growth is coupled not only with higher N/C and P/C ratios, but also with lower N/P ratios in many heterotrophic organisms, including bacteria 29 . Table 2 shows the microalgal mean daily productivity, sewage pollutant removal efficiencies and CH 4 content in biogas under three cultures and various C/N ratios. The behavior of the three selected cultures varied even under identical environmental conditions and media. In both the mono-algae and the algal-fungal culture, biomass grew faster in the C5N1-COD200 treatment than in the other treatments (p < 0.05). In the present study, biomass productivity of the algal-fungal culture was higher than the other two cultures. The highest biomass productivity (0.44 g L −1 d −1 , Table 2) for algal-fungal culture was found in C5N1-COD200, although there was no differences among other C/N ratios (p < 0.05).

Pollutants removal efficiencies.
Pollutants uptake by the three selected cultures significantly contributed to COD, TN and TP removal from the wastewater. Figures 1-3 indicated the effects of various influent C/N ratios and culture methods on COD, TN, and TP removal efficiencies using microalgal-based technologies. All the cultures studied efficiently removed pollutants (COD, TN, and TP) from the synthetic domestic sewage. Pollutant removal efficiencies fluctuated for the three algae source culture during the operational period, and the variation tendencies of pollutant removal were similar under different influent C/N ratios. The COD removal efficiencies varied between 50.06% and 89.47% for the microalgal monoculture (Fig. 1a), between 52.36% and 93.87% for the microalgal-fungal co-culture (Fig. 1b), and between 50.35% and 93.78% for the algal-bacterial co-culture ( Fig. 1c) under different COD or TN level treatments. The TN removal efficiency variations for the microalgal monoculture ranged from 50.26% to 90.69% (Fig. 2a), for the microalgal-fungal co-culture from 54.25% to 94.28 % (Fig. 2b), and for the algal-bacterial co-culture from 49.23% to 93.97% (Fig. 2c). The TP removal efficiencies for the microalgal monoculture varied between 42.17% and 89.57% (Fig. 3a), for the microalgal-fungal co-culture between 49.36% and 93.77% (Fig. 3b), and for the algal-bacterial co-culture between 47.14% and 92.54% (Fig. 3c). It is noteworthy that after the first day of the assays, pollutant concentrations (in terms of COD, TN, and TP)  Table 1. Means ± standard deviations of the physico-chemical parameters of influent and effluent for domestic sewage of three different methods of treatments at different influent C/N ratio.
suddenly dropped. This initial drop implied a COD removal of up to 50.06%, TN removal of up to 49.23%, and TP removal of up to 42.17% in all treatments and was attributed to the adsorption of the microalgae cell walls and subsequent assimilation. The highest pollutant removal efficiencies were observed on day 6 (Figs 1a-3a), day 8 (Figs 1b-3b), and day 7 (Figs 1c-3c) by mono-algae culture, algal-fungal culture, and algal-bacterial culture, respectively. Afterwards, removal efficiencies decreased until the end of the experimental period. This decrease was probably due to the accumulation of metabolic waste, which ultimately led to cell death 20 . Therefore it is suggested that the cultivation times of 6, 8, and 7 days for mono-algae culture, algal-fungal culture, and algal-bacterial culture, respectively, are needed to achieve the highest pollutants removal efficiencies. In carbon addition treatments, the C5N1-COD200 treatment (69.34%) had a higher TP removal efficiency than the C2.5N1-COD100 (62.91%) and C10N1-COD400 treatments (63.95%) (p < 0.05), but no significant difference (p > 0.05) was observed in removal efficiencies for COD and TN among C variation treatments by the mono-algae culture. For the algal-fungal culture, the C5N1-COD200 treatment had a higher TN removal efficiency (79.35%) than C2.5N1-COD100 (75.08%) and C10N1-COD400 (75.54%) (p < 0.05), but the influent C/N ratios showed no statistically significant effects (p > 0.05) on COD removal. For the algal-bacterial culture, the highest TN (79.48%) and TP (69.34%) removal efficiencies were achieved by C5N1-COD200 (p < 0.05), and the C10N1-COD400 treatment had a lower COD removal efficiency (71.60%) than the C2.5N1-COD100 (74.79%) and C5N1-COD200 treatments (77.41%) (p < 0.05).
As shown in Table 3, the interaction of treatment methods and influent C/N ratios as well as the treatment methods significantly influenced pollutants removal efficiencies (p < 0.05). Only TN and TP removal efficiency were significantly affected (p < 0.05) by influent C/N ratios. This shows that appropriate selection of treatment methods and influent C/N ratios is a simple and effective strategy to increase pollutants removal efficiencies.
Carbon is a basal element for microalgal growth, contributing to up to about 50% of microalgal biomass 35 . A previous study accessed the carbon mass balance and found that assimilation into biomass was the main carbon removal pathway 20 . Dissolved nitrogen and phosphorus in wastewater could be efficiently removed through  Table 2. Mean values ± SD of the removal efficiency of biogas CO 2 and pollutants removal of different C/N ratio at three different methods of treatments. Values with different superscript letters in the same column for the same method of treatments indicate significant differences at p = 0.05 according to Duncan's multiple range tests.
continuous microalgal growth. Total N was mainly reduced via microalgal assimilation, as algae cells require nitrogen for protein, nucleic acid, and phospholipid synthesis 9 . Previous work has documented that in wastewater, simple organic nitrogen, including urea and amino acids, can be assimilated by microalgae 15 . Thus, microalgae growth is essential for nitrogen removal via uptake, decay, and sedimentation 36 . On the other hand, partial loss of TN could be attributed to the physical absorption by strain complexes because of their unique structure 37 . In addition, phosphorus is an important nutrient in algal production as a constituent of phospholipids (for cell membranes) and adenosine triphosphate (to supply energy for cell functions), although it constitutes less than 1% of the biomass 38 8 . Carbon dioxide dissolving in sewage will provides a substrate for the microalgal photosynthesis. The dissolution of carbon dioxide promoted the growth of microalgae and therefore played a key role in nitrogen and phosphorus removal. Accordingly, coculture of microalgal and fungi or bacteria promotes the purification of sewage and biogas upgrading 14 . The rich carbon dioxide in the biogas slurry promotes the growth of algal bacteria and increases the biomass of algae bacteria, thus increases the absorption of N and P by algae and improes the sewage purification capacity of algae bacteria.
Several researchers have suggested that denitrification rates in a reactor depend largely upon the amounts of nitrate N and organic carbon as well as on environmental conditions such as pH, temperature, and DO concentration 39 . Here, influent pH levels (7.11-7.54) ( Table 1) were within the optimum range for denitrification reported by Paul and Clark (1989) 40 . In practice, nitrogen removal is highly dependent on pollution load levels. In this study, different influent C/N ratios resulted in different denitrification rates, and high TN removal efficiency occurred only if the C/N ratio was 5:1. Consequently, influent C/N ratio plays a crucial role in wastewater treatment 39 . Some scholars have suggested that influent C/N ratio is a domain value, rather than a specific value, because when the experimental influent C/N ratio approaches this value, optimal pollutant removal efficiencies can be achieved 27 . In this research, the balance of carbon and nitrogen nutrient sources in the influent affected the growth of the culture, thereby affecting the nutrient removal efficiencies of the microalgae. On the other hand, the synthetic wastewater used in this research contained carbamide; a high C/N ratio might thus inhibit the removal of ammonia nitrogen due to the competition for DO demand. Therefore, when carbon sources were lacking (C:N = 2.5:1) or nitrogen sources were insufficient (C:N = 10:1), pollutants removal decreased. Considering the combined removal efficiencies for all pollutants, the optimal C/N ratio in this research was 5:1. These results are in agreement with the findings of Yan et al. 26 , who reported that influent C/N ratio significantly affected nutrient removal efficiency and that the highest removal effect was found with a medium influent C/N ratio. Influent C/N ratio also affected effluent phosphorus concentrations, and at C/N 5:1, all treatment systems reached their highest TP removal efficiencies. In the nitrogen addition treatments, the highest average removal efficiencies for the three algae source culture occurred at the C/N ratio of 5:1 ( Table 2). These results could be explained by the combined carbon and nitrogen effects for removal of TP. Overall, the results showed that appropriate control of carbon and nitrogen input were necessary to achieve the efficient phosphorus removal.
In the present study, among the three selected cultures, the algal-fungal culture showed the highest pollutants removal efficiencies (COD, TN and TP). The sphere structure of the fungus-algae complex was stable and did not easily break into small pieces (data not shown), which might partially explains the high pollutants removal efficiency. According to the references 14, 41 , when fungi associated with microalgae grew in wastewater, the fungus was pelletized with microalgae cells through bioflocculation and non-bioflocculation. The coagulative mechanism is spore coagulation resulting in accumulation of pellets. The non-coagulative mechanism includes that the spores germinate into hyphae and then intertwines into pellets. In the symbiotic system of algae-fungi/bacteria, on one hand, the extracellular metabolites produced and secreted by the microalgae can be effectively taken up by the surrounding fungi/bacteria for the growth and reproduction. On the other hand, the bacteria in the metabolic process not only can produce the necessary nutrients and growth factors for the growth of microalgae, but also can directly or indirectly regulate the growth environment of microalgae. This forms a mutually beneficial symbiotic relationship 42 . The intergrowth of microalgae and fungi enhances the specific surface area of algae-fungi symbionts and thus the nutrient intake capacity 43 . According to Table 2, coculture of microalgae and fungi resulted in higher nutrient removal as well as CO2 removal except for TN removal. The algal-bacterial culture showed higher TN removal efficiency than algal-fungi culture maybe attribute to the nitrification of bacteria in activated sludge 44 . Biogas upgrading. At the end of the experiment, the CH 4 contents (v/v) of the biogas and the CO 2 removal efficiency (%) were investigated to evaluate differences in biogas upgrade with varying influent C/N ratios for the three selected cultures (Fig. 4a-c). The results showed that the tendencies of CH 4 contents and CO 2 removal efficiencies were similar to biomass productivities of the three selected culture, as shown in Table 2.   Table 3. P-values of factors and combined effects of factors for each parameter based on analysis of variance. Influent C/N ratios: C variation treatments (C2.5N1-COD100, C5N1-COD200, and C10N1-COD400) and N variation treatments (C10N1-TN20, C5N1-TN40, and C2.5N1-TN80); treatment methods: microalgal monoculture, microalgal-fungal coculture and microalgal-activated sludge coculture (*p < 0.05).
During the experiment, biogas upgrading was affected by the initial pollutants concentrations, and the CH 4 content (v/v) increased with CO 2 contents (v/v) decreasing. The differences in the various treatments for the CH 4 contents (v/v) of the biogas mainly resulted from the variations in the CO 2 removal, because CH 4 and CO 2 contents (v/v) in biogas were significantly negatively correlated. The CH 4 contents (v/v) enriched by algal-bacterial culture in upgraded biogas in the treatments C2.5N1-COD100, C5N1-COD200, C10N1-COD400, C10N1-TN20, C5N1-TN40, and C2.5N1-TN80 were 90.58 ± 1.25%, 93.04 ± 2.01%, 89.37 ± 1.88%, 91.04 ± 2.16%, 93.25 ± 1.74%, and 88.83 ± 1.69%, respectively; The CO 2 in biogas was utilized for algal photosynthesis. As a result, the CH 4 content in biogas was increased. The CH 4 content in biogas upgraded by algal-bacterial culture was 88.8%~93.3%, which was higher than those by mono-algae culture and algal-fungal culture. The CH 4 contents (v/v) enriched through removing CO 2 by algal-bacterial culture reached the standard of the fuel (CH 4 > 90%, v/v) in the treatments C5N1-COD200, C10N1-TN20, and C5N1-TN40. Only in C10N1-COD400 and the C2.5N1-TN80, algal-fungal culture, the CH 4 contents (v/v) did not reach this standard. The highest CH 4 content (v/v) in the different C and N variation treatments was found in C5N1-TN40, when using the three cultures.
In this research, the effect of biogas upgrading followed similar trends as biomass productivity (Table 2), mainly because approximately half of the microalgal DW was CO 2 -derived carbon 45 . When the microalgae were cultivated in synthetic domestic sewage, CO 2 was used for microalgal photosynthesis. The microalgae developed a CO 2 -concentrating mechanism to adapt to the changes in the CO 2 concentration 46, 47 . Economic efficiency of the energy consumption. The economic efficiency of energy consumption on synthetic domestic sewage purification and biogas upgrading showed a similar variation ( Table 4). The highest economic efficiency of pollutants and CO 2 removal with different culture methods can be found at the C/N ratio of 5:1 with the medium TN level. This finding is corresponded to the variation of the microalgal biomass productivity. To be specific, algal-fungal culture achieved the highest economic efficiency of COD and TP removal; algal-bacterial culture achieved the highest economic efficiency of TN and CO 2 removal. The ANOVA showed that there was no significant difference in COD, TN and TP removal rates between algal-fungi and algal-bacteria cultures (p > 0.05), and there was no significant difference on energy efficiency CO 2 removal by the three cultures (p > 0.05). For these two cocultivations, the energy efficiencies of pollutant removal were higher, but energy efficiencies of CO 2 removal were lower of than that of mono-algae culture. Microalgae will grow well in the influent C/N ratio of 5:1 and improve the sewage purification, as a result, the economic efficiency of energy consumption are increased 13 . For algal-bacteria culture, due to nitrification and denitrification of activated sludge, the economic efficiency of nitrogen removal achieve higher than algal-fungal culture 39,44 .

Conclusion
Both of the microalgal culture methods and C/N ratios had significant effects on of synthetic domestic sewage purification and biogas upgrading. The medium level of C/N ratio showed higher pollutants and CO 2 removal efficiencies than low and high C/N ratios. Co-culture of C. vulgaris with G. lucidum or activated sludge in photobioreactors was more effective than mono-cultivation on removing sewage pollutants and CO 2 in biogas simultaneously according to the data in this study. Coculture of microalgae with fungi was the suitable treatment technology for wastewater purification and biogas upgrading under the C/N ratio of 5:1.

Methods
Algae sources and culture conditions. The C. vulgaris (FACHB-31) was used for sewage treatment and biogas upgrading based on its high biogas tolerance and rapid growth in high-strength wastewater 48 . The strain was purchased from the FACHB-Collection, Institute of Hydrobiology, Chinese Academy of Sciences. The BG-11 culture media was prepared for growing the microalgal culture 49 , with initial pH adjusted to 6.9. The culture C/N ratio Economic efficiency (USD −1 ) COD  Algal-fungal culture conditions. Based on the preliminary experiment results, we found that Ganoderma lucidum (G. lucidum, 5.765) achieved high growth rate and high performance of pelletization with C. vulgaris. Therefore, the Ganoderma lucidum stain obtained from China General Microbiological Culture Collection Center was selected for this study. An inoculum was prepared by inoculating 100 mL of a synthetic medium with 25 mycelial discs. The composition of synthetic medium was as follows: glucose, 10 g L −1 ; NH 4  Algal-bacterial Culture conditions. The synthetic domestic sewage was inoculated with 1L cultured microalgal strain and 200 mL activated sludge. The total suspended solid (TSS) of microalgae and activated sludge were 0.75 g TSS L −1 and 4.14 g TSS, respectively. The activated sludge came from a wastewater treatment plant in Jiaxing, Zhejiang, China. The light intensity and temperature were the same as the algal-fungi culture 20 . Photobioreactor. Two interconnected 16.8 L (individual) glass cylinder blocks with a height of 0.6 m and a diameter of 0.2 m were used as a photobioreactor (Fig. 5). The reactors were hermetically sealed with rubber stoppers. The sampling outlet consisted of a plug and rubber gasket. The synthetic domestic sewage was pumped from the right to the left cylinder block of the photobioreactor in one time. The raw biogas was blown into the photoreactors from the raw biogas inlet until the air in the headspace was expelled.
Biogas was obtained from a farm biogas plant in JiaYuan Green Meadow. Prior to the experiments, the biogas was pretreated via chemical absorption to reduce the H 2 S content less than 0.005% (v/v). The raw biogas mainly  (Fig. 1). The initial dry weight (DW) of the three selected culture was about 90.21 ± 6.39 mg L −1 in all samples to obtain similar initial biomass concentrations. The temperature and light/dark cycle of microalgal for wastewater treatment was the same as the microalgal mono-cultivation. Under different influent C/N ratios, growth rate and pollutants removal efficiencies of the three algal source cultures and their roles in biogas upgrading were determined. Gas samples (100 mL) were drawn daily at the sampling outlet to monitor CO 2 and CH 4 concentrations. By using a 15-mL syringe, the synthetic domestic sewage was sampled daily at 8:00 a.m. through the sampling port of the photobioreactor, to monitor the COD, TN, and TP concentrations.
All treatments were performed in triplicate. The cultures were sampled and analyzed daily at 8:00 a.m. and mean values were calculated for each time and culture.
Sampling and analyses. The DW of the three algal source cultures was measured according to the reference 19 . Daily biomass productivity (P, g L −1 d −1 ) was calculated using Equation (1): where D i is the biomass concentration (g L −1 ) at time t i (d) and D 0 is the initial biomass concentration (g L −1 ) at t 0 (d).
The culture filtrates were analyzed for COD, TN, and TP concentrations by using standard methods 52 . The pH value and dissolved oxygen (DO) was measured using a pH (Orion 250 Aplus ORP Field Kit, USA) and oxygen probes (Model 862 Aplus, USA). Pollutants (COD, TN, and TP) removal efficiency was calculated as follows: i 0 where R is the pollutant removal efficiency (%), C 0 and C i are the pollutant concentrations in the initial synthetic domestic sewage and in the filtrates of the cultures (mg L −1 ), respectively. The contents (v/v) of CO 2 , H 2 S, CH 4 , and O 2 in the biogas were measured using a circulating gas analyzer 50 . The economic efficiency of the energy consumption for nutrient removal in sewage and CO 2 removal in biogas were calculated by Eq.   where E is the energy consumption for nutrient removal in sewage and CO 2 removal in biogas, USD −1 ; R is the removal efficiency in Eq. (2), %; k stands for the electric power charge per unit of energy consumption, USD kw −1 h −1 ; T is the illumination time according to photoperiod, h; P is the LED electrical power consumption, W. The electric power charge per unit of energy consumption k is around 0.08826 USD kw −1 h −1 in local after conversion 13 .
Statistical analyses. All statistical analyses were performed using the package SPSS (SPSS 2013). A one-way analysis of variance (ANOVA) was used to test the statistical differences of the 6 C/N ratios for the same microalgae cultures. Duncan's multiple range tests was employed to further test for significant differences among the treatments with different C/N ratios. A two-way ANOVA was used to test for differences among the effects of influent C/N ratios, algae sources, and the interaction between any two of these factors on treatment performance. The threshold for statistical significance was set at p = 0.05. Error bars in the figures showed the standard deviation with n = 3.